1. Field of the Invention
The present invention relates generally to a rotational position sensor and, more specifically, to a rotational position sensor for determining the angular position of a shaft in a way that avoids the possibility of restricting the motion of the shaft due to jamming of the sensor.
2. Description of the Prior Art
Various types of rotational position sensors are known to those skilled in the art. Certain angular position sensors incorporate a rotatable magnet that moves in coordination with a shaft whose angular position is to be measured. The rotating magnetic field of the magnet is sensed by a magnetically sensitive component, such as a Hall effect device or a magnetoresistive component. In many types of rotational position sensors, the magnet is attached to the shaft and rotates relative to a stationary portion of the sensor.
If the rotatable magnet becomes jammed and its rotatability is affected, the shaft whose angular position is being measured may be restricted from further movement. In certain situations, this restriction due to jamming of the sensor can be significantly deleterious. As an example, if the rotational position sensor is used to determine the angular position of a throttle plate shaft of an automobile's throttle body assembly, control of the engine's operation can be seriously affected and can possibly result in a dangerous situation. For example, if the driver of an automobile depresses the accelerator and causes the automobile's engine to increase its speed, the return of the engine to its idle speed when the driver removes the pressure from the accelerator peddle is critically important. Otherwise, the engine can continue to accelerate and an exceedingly dangerous condition can be created.
Jamming of the rotatable portion of a rotational position sensor within the stationary portion of the rotational position sensor can occur as a result of several conditions. One possible cause of this type of jamming is the formation of ice in the region between the rotatable and stationary portions of the angular position sensor. If ice forms within the region between the rotatable and stationary portions of the sensor, it is possible that the ice can retain the rotatable position sensor and its attached shaft in a position that will result in the dangerous condition described above. It would therefore be advantageous if a rotational position sensor could be provided which allows the shaft to return to its neutral position even if a rotatable portion of the rotational position sensor is jammed with respect to the stationary portion of the sensor.
As will be described in greater detail below, certain rotational position sensors have been developed which incorporate a lost motion component in the attachment between the shaft being monitored and the rotatable portion of the sensor. These types of lost motion attachments often require the use of an associated spring mechanism. However, the inclusion of a spring between the rotatable and stationary portions of the sensor in known rotational position sensors results in other problems that disadvantageously affect the operation of the angular position sensor.
U.S. Pat. No. 5,164,668, which issued to Alfors on Nov. 17, 1992, discloses an angular position sensor that has decreased sensitivity to shaft position variability. The angular position sensor is provided with first and second pole pieces that extend from regions proximate a rotatable magnet to regions proximate a magnetically sensitive device. The pole pieces provide defined magnetic paths of lower reluctance that confine the lines of flux extending between the rotatable magnet and the magnetically sensitive device. The placement of the rotatable magnet between first and second pole pieces segments of the invention significantly reduces the sensitivity of the sensor to variations in position of the rotatable magnet and therefore increases the reliability of the measurement system. This reduced sensitivity inhibits the degradation of operational accuracy that could otherwise be caused by inaccuracies in the magnet's shaft position, large tolerances in the dimensions of the shaft diameter and the bearing diameter and variable location of the shaft because of excessive bearing wear.